Version bump to 0.5.1
Version bump to 0.5.1

Includes:

* support for inheritance in the Mustache template
* a compiler for the liquid tempting language
* updates for all compilers to include the proposition header

froddd/govuk_template,bitzesty/govuk_template,alphagov/govuk_template,quis/govuk_template,daveraine/govuk_template,froddd/govuk_template,quis/govuk_template,quis/govuk_template,alphagov/govuk_template,jonheslop/govuk_template,quis/govuk_template,alphagov/govuk_template,robinwhittleton/govuk_template,daveraine/govuk_template,bitzesty/govuk_template,daveraine/govuk_template,jonheslop/govuk_template,froddd/govuk_template,bitzesty/govuk_template,froddd/govuk_template,jonheslop/govuk_template,robinwhittleton/govuk_template,alphagov/govuk_template,jonheslop/govuk_template,jonheslop/govuk_template,quis/govuk_template,robinwhittleton/govuk_template,froddd/govuk_template,robinwhittleton/govuk_template,daveraine/govuk_template,robinwhittleton/govuk_template,alphagov/govuk_template