update .travis.yml to download latest sbt-extras while waiting for tr…

…avis folks

travis-ci/travis-ci#9816
travis-ci/travis-build#1423
travis-ci/travis-cookbooks#987

.travis.yml

@@ -8,6 +8,7 @@ language: generic
 # Caching so the next build will be fast too.
 cache:
   directories:
+  - $HOME/.local/bin
   - $HOME/.stack
   - $HOME/.ivy2/cache
   - $HOME/.sbt
@@ -22,6 +23,7 @@ before_install:
 # Download and unpack the stack executable
 - mkdir -p ~/.local/bin
 - export PATH=$HOME/.local/bin:$PATH
+- curl -Ls https://git.io/sbt > ~/.local/bin/sbt && chmod 0755 ~/.local/bin/sbt
 - travis_retry curl -L https://www.stackage.org/stack/linux-x86_64 | tar xz --wildcards --strip-components=1 -C ~/.local/bin '*/stack'
 
 install: