travis: test on both osx and linux

remove apt-install

install gnu-sed on mac

clear testing section

No Need to install coreutils, we already mention gnu-sed.

Author: stevemao

.travis.yml

@@ -1,7 +1,10 @@
 language: bash
 before_install:
   - git clone --depth 1 https://github.com/sstephenson/bats.git
-  - sudo apt-get update  -qq
-  - sudo apt-get install -y coreutils
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update         ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install gnu-sed; fi
 script:
   - ./bats/bin/bats test/*.bats
+os:
+  - linux
+  - osx

readme.md

@@ -86,19 +86,19 @@ Pull requests quite welcome.
 
 ## Hacking and Testing
 
+Install [bats](github.com/sstephenson/bats#installing-bats-from-source), the Bash automated testing system we are using.
+
 ```sh
 # fork and clone the repo. then…
-
 git submodule update --init
-brew install entr coreutils
-# install bats: github.com/sstephenson/bats#installing-bats-from-source
 ```
 
 ```sh
 # Run the test suite once:
 bats test/diff-so-fancy.bats
 
 # Run it on every change with `entr`
+brew install entr
 ls --color=never diff-so-fancy test/*.bats | entr bats test/*.bats
 ```