Drop CI for the i686-apple-darwin target

32-bit Apple targets are no longer fully supported:
https://blog.rust-lang.org/2020/01/03/reducing-support-for-32-bit-apple-targets.html

Author: teskje

.travis.yml

@@ -58,8 +58,6 @@ matrix:
     # - env: TARGET=x86_64-unknown-linux-musl
 
     # OSX
-    - env: TARGET=i686-apple-darwin
-      os: osx
     - env: TARGET=x86_64-apple-darwin
       os: osx