using RCTExecuteOnMainQueue instead of RCTExecuteOnMainThread, as the function has been removed in react-native 0.42

Since React Native 0.35.0-rc.0 this parameter to imageloader is required.

This change adds support for RN 0.34.0-rc0.

Related changes in RN:
https://github.com/facebook/react-native/commit/1a7c231ef35812e8d4a79cd2482c69b10c52dc4a
https://github.com/facebook/react-native/commit/a718f14af7fee54457f7f89d2a89efb2f61a28ef#diff-eb2d634923bec6e03e2ba8558ff3c3b8

Fixes https://github.com/ProjectSeptemberInc/gl-react-native/issues/71

see https://github.com/ProjectSeptemberInc/gl-react/issues/60

Use with https://github.com/ProjectSeptemberInc/gl-react/commit/0253046f326d4e4d6f088a82b81f2daa955623b9

- format: "base64" || "file"
- type: "png" || "jpeg"/"jpg" || "webm" (android only)
- quality: from 0 to 1 (iOS support only with type=jpg/jpeg)
- filePath: for format=="file" you must provide a file path (you can use react-native-fs to get directory constants)

Attempt to fix #38

See https://github.com/ProjectSeptemberInc/gl-react/issues/38