Use the public TensorFlow API to access SaverDef

[mrry]

The current code accesses SaverDef using an obsolete private API. This change makes skflow compatible with TensorFlow 0.7.0 and up.

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[codecov-io]

Current coverage is 90.38%

Merging #127 into master will not affect coverage as of 5075766

@@            master    #127   diff @@
======================================
  Files           27      27       
  Stmts         1061    1061       
  Branches       164     164       
  Methods          0       0       
======================================
  Hit            959     959       
  Partial         50      50       
  Missed          52      52       

Review entire Coverage Diff as of 5075766

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[mrry]

I signed it!

[googlebot]

CLAs look good, thanks!

[terrytangyuan]

Thanks!