ci: drop official support for rustc<1.7.0

As time goes on, the likelihood that a user will need an older
version of rust decreases (users with existing apps may continue
to use older versions of the library).

For now, we are dropping support for versions older than 1.7.0.

Signed-off-by: Paul Osborne <[email protected]>

Author: posborne

.travis.yml

@@ -1,11 +1,6 @@
 language: rust
 sudo: false
 rust:
-  - 1.2.0
-  - 1.3.0
-  - 1.4.0
-  - 1.5.0
-  - 1.6.0
   - 1.7.0
   - stable
   - beta