[FEA] upgrade cuml to c++14

[teju85]

Is your feature request related to a problem? Please describe.
cudf is already at C++14. Except for the exec_policy code simplification as described in PR #247 , I don't see much of C++14-specific use-case in cuML, for now. Also, I don't see any reason not to upgrade to 14. Thus, for an overall uniform RAPIDS environment, it does make sense to upgrade cuML to C++14.

Describe the solution you'd like
We need to upgrade our code to C++14 for cuML. However, IMO, this can be a lower priority task, aimed for branch-0.7 timeframe.

Describe alternatives you've considered
None.

Additional context
-NA-

[cjnolet]

@teju85, Given the amount of work we have on our plates for 0.7, can we push this to 0.8?

[teju85]

With the amount of work we have, I'd even go further to claim that we should probably revisit this item in 0.9!