build with ghc-8.6.3 + tests fix#22

Merged

NCrashed merged 5 commits intopostgres-haskell:masterfrom

Jan 29, 2019

Commits on Jan 23, 2019

build with ghc-8.6.3
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build for 8.2.2 & 8.4.4 with cpp
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travis env for 8.2/8.4/8.6
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Commits on Jan 24, 2019

tests fix, removed builds for 7.10 / 8.0, pg 10, travis xenial base image
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char type allows only 7bit values, tests for char encode/decode fixed
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