Bump beefcake dependency

[dch]

While porting this to FreeBSD \o/ I found that the beefcake dependency is well out of date, and at least on FreeBSD 12.0-BETA3 the tests fail when run under bundler:

  - should survive local close
  - should raise Riemann::Client::Unsupported exception on query

Bacon::Error: "warning".==("ok") failed
	spec/client.rb:264:in `block (2 levels) in <top (required)>': Riemann::Client (UDP transport) - should survive inactivity
	spec/client.rb:251:in `block in <top (required)>'
	spec/client.rb:231:in `<top (required)>'

31 specifications (71 requirements), 1 failures, 0 errors

After this commit, tests pass and the latest version of beefcake is used:

> bundle exec bacon spec/client.rb
NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01.
Gem::Specification#has_rdoc= called from /tmp/riemann-ruby-client/riemann-client.gemspec:21.
Riemann::Client (TCP transport)
  - should yield itself to given block
  - should close sockets if given a block that raises
  - should be connected after sending
  - should send longs
  - should send doubles
  - should send custom attributes
  - should send a state with a time
  - should send a state without time
  - should query states
  - []
  - should query quickly
     1603.22 queries/sec (0.62ms per query)

  - should be threadsafe
     3711.72 inserts/sec (0.27ms per insert)

  - should send a state
  - should survive inactivity
  - should survive local close

Riemann::Client (UDP transport)
  - should yield itself to given block
  - should close sockets if given a block that raises
  - should be connected after sending
  - should send longs
  - should send doubles
  - should send custom attributes
  - should send a state with a time
  - should send a state without time
  - should query states
  - []
  - should query quickly
     2215.34 queries/sec (0.45ms per query)

  - should be threadsafe
     9393.14 inserts/sec (0.11ms per insert)

  - should send a state
  - should survive inactivity
  - should survive local close
  - should raise Riemann::Client::Unsupported exception on query

31 specifications (71 requirements), 0 failures, 0 errors

• https://reviews.freebsd.org/D17710
• 5eed965

I'm happy to remove the bonus .gitignore commit if that's not helpful.