Don't use build cache (a.k.a. task output caching) under Travis CI

The performance improvement offered by the build cache is modest when
run by Travis CI.  This is probably because Travis CI does not keep
the cache on any local machine.  Instead, Travis CI uploads the build
cache across a network at the end of each run, then restores the cache
across the network at the start of the next run.  So in many cases
we're simply trading network access to original downloads for network
access to the cache image.

Furthermore, it's probably a better test to perform Travis CI testing
jobs from something closer to a clean slate.  We really want to know
whether everything builds and passes tests correctly starting from
nothing.  We don't want to risk accidentally thinking something would
work simply because we have a cached copy of what it did when it
worked previously.

travis/install-gradle

@@ -1,3 +1,3 @@
 # -*- mode: sh; sh-shell: sh -*-
 
-./gradlew --continue assemble
+./gradlew --continue --no-build-cache assemble

travis/script-gradle

@@ -5,4 +5,4 @@ case "$TRAVIS_OS_NAME" in
   (osx) headless='' ;;
 esac
 
-$headless ./gradlew --continue --stacktrace build javadoc lintGradle
+$headless ./gradlew --continue --no-build-cache --stacktrace build javadoc lintGradle