Update the build

[Jawnnypoo]

Just wanted to make this easier for the next person who pulls down the repo and wants to build.

Bump build tools, target sdk, and target Java 7 so that it does not break builds which try to compile with Java 8 (which is what Android Studio was trying to compile with by default for me)

This will also allow for users to use newer features like instant run when developing. Always a plus!

[Jawnnypoo]

allprojects {
     sourceCompatibility = JavaVersion.VERSION_1_7
     targetCompatibility = JavaVersion.VERSION_1_7
 }

Do you guys have any ideas as to why this is needed or why this was not an issue before? It may have just been an issue for me, since I have JDK 8 installed and maybe Android Studio was just compiling to the latest. Regardless, I think it would be nice to have this in here explicit for anyone else who might run into this issue. I suppose others were encountering it, after seeing issue #71

[Jawnnypoo]

@willbailey Any chance of reviewing this? It is an issue any developer who wants to make updates will probably run into.

Case in point, #77 already repeats some of the work done here.

Is this library still being maintained?