Fix compatibility with RSpec 2.x

The latest version breaks compatibility with older versions of RSpec.

::RSpec.configuration.start_time= was added in RSpec 3.0.0.

This method doesn't exist in RSpec 2.x, so the binstub fails with  `spring-commands-rspec-1.0.3/lib/spring/commands/rspec.rb:17:in `call': undefined method `start_time=' for #<RSpec::Core::Configuration:0x007f6d388dcf00> (NoMethodError)`

This PR checks if the start_time= method exists before calling it.

The issue Load time incorrect with Spring #18 doesn't occur with RSpec 2.x so no additional fix is necessary for that.

Other people are having this issue and are forking the repo to fix it. See https://github.com/lihanli/spring-commands-rspec
Perhaps we could just check `defined?(::RSpec.configuration.start_time)`?