ci: drop support for rust <1.6.0

These don't pass as a result of vec_map not supporting these
versions (depended on by clap).  Not supporting older versions
of rust is probably fine for this project.

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

Author: posborne

.travis.yml

@@ -2,10 +2,6 @@ language: rust
 sudo: false
 
 rust:
-  - 1.2.0
-  - 1.3.0
-  - 1.4.0
-  - 1.5.0
   - 1.6.0
   - stable
   - beta