Incompatibilities between BSD sed and GNU sed

Whereas GNU sed's -i/--in-place option optionally takes an argument
(which is a suffix, and means to modify the input file in place if
you omit an argument), BSD sed's -i option always takes an argument
even if it's an empty string (e.g., -i '').

My solution is install GNU sed using Homebrew on macOS build.

Note that omitting --with-default-names will prepend "g" to the
command, i.e., gsed.

[changelog skip]

.travis.yml

@@ -78,6 +78,7 @@ before_install:
     brew update || true
     brew upgrade || true
     brew update || true
+    brew install --with-default-names gnu-sed
     brew tap dahlia/homebrew-deadsnakes
     brew install python34 python35 python3 || \
       brew upgrade python34 python35 python3 || \