Bump Pkg

[KristofferC]

Some bugfixes:

• Allow using current env when testing: Tests fail when run through Pkg Pkg.jl#306
• Throw error when a package fails to build: Build failures doesn't throw an error. Pkg.jl#297
• Fixes adding packages with REQUIRE files that specified version bounds as e.g. 0.1.0 0.1.0+. adding Deprecations.jl fails with unsatisfiable requirement Pkg.jl#293
• Fix a problem when the stdlib finder for old packages errors when encountering files with weird permissions. stdlib finder for old packages tries to walk into paths it doesn't have access to Pkg.jl#314
• CommandError didn't print properly due to no show error being defined
• Fixed a bug that made fixed packages (checked out packages) not have their name in the resolver backtrace. adding Deprecations.jl fails with unsatisfiable requirement Pkg.jl#293

Other changes:

• Only updates the registry on the first add of the session. Updating on every add was too much and it is always possible to just use update.
• No longer show the backtrace from a resolver error when using the Pkg REPL.

[KristofferC]

Actually, we should perhaps think a bit about holding the fix for JuliaLang/Pkg.jl#297 until JuliaLang/Pkg.jl#165 is resolved.

Edit: Dropped the fix for that for now.