allow use of compilers with higher minor version

[d10r]

fixes #61

[codecov-io]

Codecov Report

Merging #66 into devel will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##           devel    #66   +/-   ##
====================================
  Coverage    100%   100%           
====================================
  Files          5      5           
  Lines        160    160           
  Branches      26     26           
====================================
  Hits         160    160

Impacted Files	Coverage Δ
contracts/ERC777BaseToken.sol	100% <ø> (ø)	⬆️
contracts/examples/ExampleTokensRecipient.sol	100% <ø> (ø)	⬆️
contracts/examples/ReferenceToken.sol	100% <ø> (ø)	⬆️
contracts/examples/ExampleTokensSender.sol	100% <ø> (ø)	⬆️
contracts/ERC777ERC20BaseToken.sol	100% <ø> (ø)	⬆️

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