Stop building on older Rusts than 1.17.0

While the library still works, all targets can't be tested since the
"required features" feature in Cargo is absent, making it try to
compile the Rocket integration when running the test suite.

Author: mhallin

.travis.yml

@@ -7,8 +7,6 @@ rust:
   - nightly
 
   # Prevent accidentally breaking older Rust versions
-  - 1.15.0
-  - 1.16.0
   - 1.17.0
   - 1.18.0