[WIP] Get tests passing on Julia v0.4-v0.6

[jgoldfar]

Will require JuliaDatabases/DBInterface.jl#16 to be merged to get tests passing on v0.6. Probably worth registering DBI and tagging a version. Intended to eventually fix #52 and will close #51

[coveralls]

Coverage decreased (-63.9%) to 11.628% when pulling 2ec6e62 on jgoldfar:get-tests-passing-v0.4-v0.6 into a29b45f on JuliaDB:master.

[jgoldfar]

By the way, I've thought about the discussion in #48 (comment) which I believe this will also suffer from (one of the reasons for the WIP tag) but the referenced functionality does not seem to be tested anywhere, so making sure an implementation is working would be tricky. Is there a test case?

[iamed2]

A good test case: make a string type that doesn't rely on an array of bytes and call pgdata with that.

[codecov-io]

Codecov Report

Merging #53 into master will decrease coverage by 63.93%.
The diff coverage is 7.14%.

@@             Coverage Diff             @@
##           master      #53       +/-   ##
===========================================
- Coverage   75.56%   11.62%   -63.94%     
===========================================
  Files           3        3               
  Lines         221      215        -6     
===========================================
- Hits          167       25      -142     
- Misses         54      190      +136

Impacted Files	Coverage Δ
src/types.jl	7.14% <ø> (-63.13%)	⬇️
src/dbi_impl.jl	11.97% <0%> (-65.81%)	⬇️
src/libpq_common.jl	100% <100%> (ø)	⬆️

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