Gradle: Specify version of JUnit to avoid unreproducible builds

Another reason for this change is that it stops Gradle from making
unnecessary trips to the network to verify if it has the latest version
of JUnit. ie, on my laptop, even after many successful builds, it was
still possible to get this error if disconnected from the internet:

--
$ gradle test

FAILURE: Build failed with an exception.

* What went wrong:
Could not resolve all dependencies for configuration ':core:testCompile'.
> Could not resolve junit:junit:4.+.
  Required by:
      spongycastle:core:1.50-b01
   > Failed to list versions for junit:junit:4.+.
      > Could not list versions using M2 pattern 'http://repo1.maven.org/maven2/[organisation]/[module]/[revision]/[artifact]-[revision](-[classifier]).[ext]'.
         > Could not GET 'http://repo1.maven.org/maven2/junit/junit/'.
            > repo1.maven.org
--

1 files changed, 1 insertions, 1 deletions

diff --git a/build.gradle b/build.gradle
index 2b41765f..fcda0e56 100644
--- a/build.gradle
+++ b/build.gradle
@@ -11,7 +11,7 @@ subprojects {
     }
 
     dependencies {
-        testCompile group: 'junit', name: 'junit', version: '4.+'
+        testCompile group: 'junit', name: 'junit', version: '4.11'
     }
 
     sourceCompatibility = 1.5