test(Travis): Remove old tmux versions

Save travis some vm time by removing old versions. I'm not going to go
back to support them, so no point in knowing if they work/break.

.travis.yml

@@ -7,42 +7,15 @@ rust:
 matrix:
   allow_failures:
     - env: TMUX_VERSION=master
-    - env: TMUX_VERSION=1.9a
+    - env: TMUX_VERSION=2.7
   exclude:
     - os: osx
       env: TMUX_VERSION=2.7
-    - os: osx
-      env: TMUX_VERSION=2.6
-    - os: osx
-      env: TMUX_VERSION=2.5
-    - os: osx
-      env: TMUX_VERSION=2.4
-    - os: osx
-      env: TMUX_VERSION=2.3
-    - os: osx
-      env: TMUX_VERSION=2.2
-    - os: osx
-      env: TMUX_VERSION=2.1
-    - os: osx
-      env: TMUX_VERSION=2.0
-    - os: osx
-      env: TMUX_VERSION=1.9a
-    - os: osx
-      env: TMUX_VERSION=1.8
 
 env:
   - TMUX_VERSION=master
   - TMUX_VERSION=2.8
   - TMUX_VERSION=2.7
-  - TMUX_VERSION=2.6
-  - TMUX_VERSION=2.5
-  - TMUX_VERSION=2.4
-  - TMUX_VERSION=2.3
-  - TMUX_VERSION=2.2
-  - TMUX_VERSION=2.1
-  - TMUX_VERSION=2.0
-  - TMUX_VERSION=1.9a
-  - TMUX_VERSION=1.8
 
 os:
   - linux