Document Viewer: attempt to fix failing build of 2.7.6

Upstream forgot to update gradleVersion in build.gradle, which seems to be causing the build to fail: https://f-droid.org/wiki/index.php?title=org.sufficientlysecure.viewer/lastbuild_2767&redirect=no

Quick hack with sed. I fixed this upstream, so the sed hack can be removed next release.