gradlew: bump to 2.12

Also remove the 2.11 restriction on the root build.gradle file. It's
unnecessary, as the Android plugin will already error if the version is
too old. This means that the build will work on any version that is new
enough, which should be 2.10-2.12 at the time of writing.

build.gradle

@@ -7,7 +7,3 @@ buildscript {
         classpath files('libs/gradle-witness.jar')
     }
 }
-
-task wrapper(type: Wrapper) {
-    gradleVersion = '2.11'
-}

gradle/wrapper/gradle-wrapper.properties

@@ -1,7 +1,6 @@
-#Mon Feb 15 16:30:29 GMT 2016
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.11-bin.zip
-distributionSha256Sum=8d7437082356c9fd6309a4479c8db307673965546daea445c6c72759cd6b1ed6
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.12-bin.zip
+distributionSha256Sum=e77064981906cd0476ff1e0de3e6fef747bd18e140960f1915cca8ff6c33ab5c