Allow PE_VER to set @build.pe_version

Currently we only set @build.pe_version with the value of ENV['PE_VER'] in
pl:load_extras. We should set this always if present, otherwise it breaks
pe:local_deb.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #132 from MosesMendoza/allow_pe_ver_envvar

Allow PE_VER to set @build.pe_version