Do not check context to determine thinking sphinx version

It clashes with rspec 3.x `context` method, so instead let's use `ThinkingSphinx.sphinx_pid`

With thinking-sphinx v3:
```
[1] pry(main)> ThinkingSphinx.context
=> RSpec::ExampleGroups::Anonymous
[2] pry(main)> ThinkingSphinx.sphinx_pid
NoMethodError: undefined method `sphinx_pid' for ThinkingSphinx:Module
from (pry):2:in `<main>'
[3] pry(main)> 
```

@pat