fix preprocessor guards for getContextPlatformVersion

[bashbaug]

fixes #108

This change corrects the preprocessor guards around detail functions to get the OpenCL version from a platform, device, or context.

• The preprocessor guard for CL_HPP_TARGET_OPENCL_VERSION needs to be the smallest value that callers of these functions test for. This is correctly 120 for current usages of these functions.
• The preprocessor guard for CL_HPP_MINIMUM_OPENCL_VERSION needs to be the largest value that callers of these functions test for. This needs to be 200, for example for command queue properties.

Debatably the preprocessor guards around these functions should be removed completely, but at least this way things won't be broken while we figure out if this is the right thing to do or not.

[bashbaug]

Need to file an issue (or add a test) to ensure these types of failures are caught in the future.

[bashbaug]

I've added a test to ensure these issues will be caught in the future - see https://github.com/KhronosGroup/OpenCL-CLHPP/pull/252. These tests fail currenlty, but will pass once this PR is merged.

[bashbaug]

Merging as discussed in the December 5th teleconference.