Downgrade to gradle 1.9 to fix fdroid builds

fdroid's build system appears to still be using gradle 1.9:

https://f-droid.org/wiki/page/org.mozilla.mozstumbler/lastbuild
```
Cleaning Gradle project...
Could not build app org.mozilla.mozstumbler due to BuildException: 'Error cleaning org.mozilla.mozstumbler:0.10.2'
==== detail begin ====
Available gradle versions: 1.4 1.6 1.7 1.8 1.9
Found 1.10 via gradle plugin version 0.8
/opt/gradle/bin/gradle: line 18: /opt/gradle/versions/1.10/bin/gradle: No such file or directory
==== detail end ====
```