Loosen up the opencensus dependency

[tpitale]

Will allow for oc_datadog to work with opencensus_phoenix and others. Resolves #5

[codecov-io]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master      #6   +/-   ##
======================================
  Coverage    67.9%   67.9%           
======================================
  Files           2       2           
  Lines          81      81           
======================================
  Hits           55      55           
  Misses         26      26

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[tpitale]

Attempting to resolve a similar issue I guess to #3 I'm hoping that ~> 0.7 works better than ~> 0.7.0, though I don't know how strict opencensus is with semver.

I guess this is also because mix is more strict with versioning.

[hauleth]

@tpitale it is quite strict, the problem is that 0.x.y do not guarantee any stability accordingly to specs.

[0x6a68]

resolution?

[0x6a68]

thanks