[gemspec] Pin colored version

When building r10k.gemspec:

    WARNING:  open-ended dependency on colored (>= 1.2) is not recommended
      if colored is semantically versioned, use:
        add_runtime_dependency 'colored', '~> 1.2'

The colored gem hasn't seen an update since 2010 so can be reasonably
pinned to an exact version.
[gemspec] Loosen version constraints on rspec and vcr

When building r10k.gemspec:

    WARNING:  pessimistic dependency on rspec (~> 3.1.0, development) may be overly strict
      if rspec is semantically versioned, use:
        add_development_dependency 'rspec', '~> 3.1', '>= 3.1.0'
    WARNING:  pessimistic dependency on vcr (~> 2.9.3, development) may be overly strict
      if vcr is semantically versioned, use:
        add_development_dependency 'vcr', '~> 2.9', '>= 2.9.3'

RSpec is very good about using semantic versioning and vcr should be
reasonable as well so we can rely on the semantic version operator to
more intelligently select versions
[maint] Pin log4r version

When building r10k.gemspec:

    WARNING:  open-ended dependency on log4r (>= 1.1.10) is not recommended
      if log4r is semantically versioned, use:
        add_runtime_dependency 'log4r', '~> 1.1', '>= 1.1.10'

log4r 1.1.10 was released in 2012 and 1.1.11 was released in 2014 but
was yanked; we can pin this version until requirements change.
[gemspec] Loosen json_pure version constraint

When building r10k.gemspec:

    WARNING:  pessimistic dependency on json_pure (~> 1.8.1) may be overly strict
      if json_pure is semantically versioned, use:
        add_runtime_dependency 'json_pure', '~> 1.8', '>= 1.8.1'

The json_pure gem hasn't been updated since 2013 but we should pick up
any improvements as they come out so we can loosen this version
restriction.
Merge branch 'maint/master/gemspec-warnings'