travis: Cache the phantomjs package downloads between builds.

This should hopefully fix the issue we've been seeing with
bitbucket.org rejecting connections from Travis CI by not needing to
connect to them.

.travis.yml

@@ -1,9 +1,11 @@
 before_install:
-   - nvm install 0.10 
+   - nvm install 0.10
 install:
   - tools/travis/setup-$TEST_SUITE
 cache:
   - apt: false
+  - directories:
+    - /srv/phantomjs
 env:
   - TEST_SUITE=frontend
   - TEST_SUITE=backend

provision.py

@@ -113,8 +113,9 @@ def main():
         PHANTOMJS_PATH = "/srv/phantomjs"
         PHANTOMJS_TARBALL = os.path.join(PHANTOMJS_PATH, "phantomjs-1.9.8-linux-x86_64.tar.bz2")
         sh.mkdir("-p", PHANTOMJS_PATH, **LOUD)
-        sh.wget("https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-1.9.8-linux-x86_64.tar.bz2",
-                output_document=PHANTOMJS_TARBALL, **LOUD)
+        if not os.path.exists(PHANTOMJS_TARBALL):
+            sh.wget("https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-1.9.8-linux-x86_64.tar.bz2",
+                    output_document=PHANTOMJS_TARBALL, **LOUD)
         sh.tar("xj", directory=PHANTOMJS_PATH, file=PHANTOMJS_TARBALL, **LOUD)
         sh.ln("-sf", os.path.join(PHANTOMJS_PATH, "phantomjs-1.9.8-linux-x86_64", "bin", "phantomjs"),
               "/usr/local/bin/phantomjs", **LOUD)