Fix CI failures by bumping rust to 1.18 and using pip2

Some of the dependencies require 1.18 now. The newer OSX images in Travis
require using pip2 (or pip3) instead of just pip. With these fixes the CI
tests now pass.

Author: tpmanley

.travis.yml

@@ -1,7 +1,7 @@
 language: rust
 sudo: false
 rust:
-  - 1.15.0
+  - 1.18.0
   - stable
   - beta
   - nightly
@@ -19,7 +19,7 @@ addons:
       - binutils-dev
 
 before_script:
-  - pip install 'travis-cargo<0.2' --user
+  - pip2 install 'travis-cargo<0.2' --user
 
 env:
   global: