let's try to build releases with musl

b/c right now it fails with:

% ldd ~/.local/bin/pretty-git-prompt
        libcurl-gnutls.so.4 => not found
        libssl.so.1.0.0 => not found
        libcrypto.so.1.0.0 => not found

Signed-off-by: Tomas Tomecek <ttomecek@redhat.com>

.travis.yml

@@ -16,7 +16,7 @@ matrix:
       env: TARGET=x86_64-apple-darwin
     - os: linux
       rust: stable
-      env: TARGET=x86_64-unknown-linux-gnu
+      env: TARGET=x86_64-unknown-linux-musl
 script:
   - hack/ci.sh
 before_deploy: