Fix Travis config for bcrypt install

The Travis build was failing to compile bcrypt due to a C++
incompatibility issue. See [https://goo.gl/BS3stK](the Travis docs) for
the source of this change.

.travis.yml

@@ -1,7 +1,15 @@
 language: node_js
 node_js:
   # Latest stable version
-- node
+  - node
+env:
+  - CXX=g++-4.8
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-4.8
 cache: yarn
 notifications:
   slack:
@@ -10,11 +18,11 @@ notifications:
     on_success: never
     on_failure: always
 before_install:
-# Copy environment variables file to prevent error
-- cp .env.example .env
+  # Copy environment variables file to prevent error
+  - cp .env.example .env
 after_success:
-# Send test coverage to Coveralls
-- "./node_modules/.bin/nyc report --reporter=text-lcov | ./node_modules/.bin/coveralls"
+  # Send test coverage to Coveralls
+  - "./node_modules/.bin/nyc report --reporter=text-lcov | ./node_modules/.bin/coveralls"
 deploy:
   provider: script
   script: scripts/deploy.sh