If Travis CI gives us multiple CPUs, we may as well use them

I believe Travis CI jobs get two CPUs by default.

Doing parallel builds regularly is also a good way to help us discover
any build race conditions we may have.  There's no guarantee that any
such races will be revealed, but even exposing them
nondeterministically is better than having no possibility of exposing
them at all.

travis/install-gradle

@@ -1,3 +1,3 @@
 # -*- mode: sh; sh-shell: sh -*-
 
-./gradlew --continue assemble
+./gradlew --continue --parallel assemble

travis/script-gradle

@@ -5,4 +5,4 @@ case "$TRAVIS_OS_NAME" in
   (osx) headless='' ;;
 esac
 
-$headless ./gradlew --continue build javadoc lintGradle
+$headless ./gradlew --continue --parallel build javadoc lintGradle