We have a crash in our application, and based on our analysis the culprit is a
race condition in RNFetchBlobNetwork initialization.

Crashing thread:

        Crashed: com.apple.root.default-qos
	0  libobjc.A.dylib                0x10416bacb objc_msgSend + 11
	1  Foundation                     0x103d3b644 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 153
	2  Foundation                     0x103c23f8c -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 61
	3  Foundation                     0x103c23f3a -[NSOperationQueue setMaxConcurrentOperationCount:] + 198
	4  prymr                          0x100d579aa -[RNFetchBlobNetwork init] (RNFetchBlobNetwork.m:112)
	5  prymr                          0x100d63a73 __65-[RNFetchBlob fetchBlob:taskId:method:url:headers:body:callback:]_block_invoke (RNFetchBlob.m:131)
	6  prymr                          0x100d6006b __85+[RNFetchBlobReqBuilder buildOctetRequest:taskId:method:url:headers:body:onComplete:]_block_invoke (RNFetchBlobReqBuilder.m:178)
	7  libdispatch.dylib              0x107511585 _dispatch_call_block_and_release + 12

While a second thread is running:

	com.apple.root.default-qos
	0  libsystem_kernel.dylib         0x107891c22 __psynch_mutexwait + 10
	1  libsystem_pthread.dylib        0x1078c6dfa _pthread_mutex_lock_wait + 100
	2  libsystem_pthread.dylib        0x1078c4519 _pthread_mutex_lock_slow + 285
	3  Foundation                     0x103d3b615 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 106
	4  Foundation                     0x103c23f8c -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 61
	5  Foundation                     0x103c23f3a -[NSOperationQueue setMaxConcurrentOperationCount:] + 198
	6  prymr                          0x100d579aa -[RNFetchBlobNetwork init] (RNFetchBlobNetwork.m:112)
	7  prymr                          0x100d63a73 __65-[RNFetchBlob fetchBlob:taskId:method:url:headers:body:callback:]_block_invoke (RNFetchBlob.m:131)
	8  prymr                          0x100d6006b __85+[RNFetchBlobReqBuilder buildOctetRequest:taskId:method:url:headers:body:onComplete:]_block_invoke (RNFetchBlobReqBuilder.m:178)
	9  libdispatch.dylib              0x107511585 _dispatch_call_block_and_release + 12

The patch just adds a dumb double-synchronization to the initialization.

remove extra space

createJSModules was removedin React Native 0.47.0 and results in the attached Build Error.
removing @Override fixes build (didn't remove function because I don't know if it should be kept for downard compatability?)

```
node_modules/react-native-fetch-blob/android/src/main/java/com/RNFetchBlob/RNFetchBlobPackage.java:23: error: method does not override or implement a method from a supertype
    @Override
    ^
1 error

Incremental compilation of 1 classes completed in 0.219 secs.
:react-native-fetch-blob:compileReleaseJavaWithJavac FAILED

FAILURE: Build failed with an exception.
```

To support the default react-native gradle version
https://github.com/wkh237/react-native-fetch-blob/pull/396

Change to jCenter because mavenCentral does not have the latest version of the package

fix