Address be_true & be_false deprecation warnings.

[erictheise]

A number of deprecation warnings related to be_true and be_false are generated when running specs:

`be_false` is deprecated. Use `be_falsey` (for Ruby's conditional semantics) or `be false` (for exact `== false` equality) instead. Called from /Users/erictheise/Projects/erictheise/poltergeist/spec/integration/session_spec.rb:242:in `block (3 levels) in <top (required)>'.
`be_true` is deprecated. Use `be_truthy` (for Ruby's conditional semantics) or `be true` (for exact `== true` equality) instead. Called from /Users/erictheise/Projects/erictheise/poltergeist/spec/integration/driver_spec.rb:197:in `block (3 levels) in <module:Poltergeist>'.
Too many uses of deprecated '`be_true`'. Pass `--deprecation-out` or set `config.deprecation_stream` to a file for full output.

These were deprecated in RSpec's move from 2.14 to 2.99.

Just hoping to contribute a little cleanup as I get my own local setup to pass all tests.