Update matrix to only include nightly to fix the tests.

Author: thehydroimpulse

.travis.yml

@@ -3,10 +3,6 @@ matrix:
   include:
   - rust: nightly
     env: TEST_SUITE=suite_nightly
-  - rust: beta
-    env: TEST_SUITE=suite_beta
-  - rust: stable
-    env: TEST_SUITE=suite_stable
 script:
 - cargo build --verbose
 - cargo test --verbose