Remove musl from build matrix

The compiler doesn't currently support musl dynamic linking:

rust-lang/rust#34987

Reconsider this removal if/when this gets fixed.

.travis.yml

@@ -23,17 +23,6 @@ matrix:
     - os: linux
       rust: stable
       env: TARGET=x86_64-unknown-linux-gnu
-    - os: linux
-      rust: stable
-      env: TARGET=x86_64-unknown-linux-musl
-      dist: trusty
-      sudo: required
-      addons:
-        apt:
-          packages: &musl_packages
-          - musl
-          - musl-dev
-          - musl-tools
 
     # Beta.
     - os: osx
@@ -42,14 +31,6 @@ matrix:
     - os: linux
       rust: beta
       env: TARGET=x86_64-unknown-linux-gnu
-    - os: linux
-      rust: beta
-      env: TARGET=x86_64-unknown-linux-musl
-      dist: trusty
-      sudo: required
-      addons:
-        apt:
-          packages: *musl_packages
 
     # Nightly.
     - os: osx
@@ -58,14 +39,6 @@ matrix:
     - os: linux
       rust: nightly
       env: TARGET=x86_64-unknown-linux-gnu
-    - os: linux
-      rust: nightly
-      env: TARGET=x86_64-unknown-linux-musl
-      dist: trusty
-      sudo: required
-      addons:
-        apt:
-          packages: *musl_packages
 
 before_install:
   - export PATH="$PATH:$HOME/.cargo/bin"