Disable solidity-coverage

This is required to update to truffle 4 and solc 0.4.18, see PR #573

.travis.yml

@@ -6,13 +6,6 @@ node_js:
   - "6"
 cache:
   yarn: true
-env:
-  -
-  - SOLIDITY_COVERAGE=true
-matrix:
-  fast_finish: true
-  allow_failures:
-    - env: SOLIDITY_COVERAGE=true
 before_script:
   - truffle version
   - yarn list