Unity: ignore mdb files and fix path to crashlytics file (it can be in other places)

Unity projects targeting the 2.0/3.5 runtime or built with mono < v5.0 generate `mdb` files, not `pdb` files.

Looks like the `crashlytics-build.properties` gets around in more than just the `StreamingAssets` folder, looking at [examples around the internets](https://github.com/auth0/sharelock-android/blob/master/app/src/main/assets/crashlytics-build.properties), so it should probably just be ignored as a filename.

Unity.gitignore

@@ -26,12 +26,14 @@ ExportedObj/
 *.booproj
 *.svd
 *.pdb
+*.mdb
 *.opendb
 *.VC.db
 
 # Unity3D generated meta files
 *.pidb.meta
 *.pdb.meta
+*.mdb.meta
 
 # Unity3D generated file on crash reports
 sysinfo.txt
@@ -41,5 +43,5 @@ sysinfo.txt
 *.unitypackage
 
 # Crashlytics generated file
-Assets/StreamingAssets/crashlytics-build.properties
+crashlytics-build.properties