define WPLANG only for WP < 4.0
add tests to check if WPLANG is defined only for WP < 4.0
Merge pull request #1524 from rodrigoprimo/wplang

`wp core config` should define WPLANG only for WP < 4.0
Don't allow active language to be uninstalled
Merge pull request #1527 from wp-cli/1516-language-active

Don't allow active language to be uninstalled
Added new subcommands wp user term and wp post term
Improved tests, formatting and backwards compatibility for post / user term
Merge pull request #1528 from spacedmonkey/commandwithterms

Support listing, updating, and removing terms associated with posts and users