Travis: Use swimm version 0.5.0, and not "latest" from GitHub API

GitHub only allows so many API calls per hour. If this is exceeded, the
travis ci build will fail.

.travis.yml

@@ -80,8 +80,7 @@ script:
 
 # verify swimm
 - cd $TRAVIS_BUILD_DIR
-- curl -s https://api.github.com/repos/swimmio/SwimmReleases/releases/latest | grep 'browser_download_url.*swimm-cli' | cut -d '"' -f 4 | wget -O swimm_cli -qi -
-- chmod +x swimm_cli
+- curl -L https://github.com/swimmio/SwimmReleases/releases/download/v0.5.0-0/swimm-cli.js --output swimm_cli
 - node swimm_cli --version
 - node swimm_cli verify