.travis.yml: Check that the dictionary is sorted (#383)

* Sort the dictionary

* .travis.yml: Check that the dictionary is sorted

To avoid committing an unsorted dictionary and having to fix it up later, check
that the dictionary is sorted in Travis-CI.

This fixes issue #310.

* Makefile: Use LC_ALL=C instead of LANG=C to set locale for sort

The POSIX manual page for sort states:

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of sort:

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base Definitions
                 volume of POSIX.1‐2008, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale categories.)

       LC_ALL    If set to a non-empty string value, override the values of
                 all the other internationalization variables.

       [...]

So, it seems safer to use LC_ALL to set the locale.