Modifying to use the libiconv-1.15

[sambitdash]

libiconv-1.14 does not build on newer versions of Ubuntu. This has been reported in the issue #16. This PR is to address the same.

[sambitdash]

The changes should not affect Windows OS.

[coveralls]

Coverage decreased (-0.3%) to 87.097% when pulling bc4ff67 on sambitdash:master into 166f5d4 on nalimilan:master.

[codecov-io]

Codecov Report

Merging #17 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   87.43%   87.09%   -0.34%     
==========================================
  Files           2        2              
  Lines         183      186       +3     
==========================================
+ Hits          160      162       +2     
- Misses         23       24       +1

Impacted Files	Coverage Δ
src/StringEncodings.jl	87.86% <0%> (-0.38%)	⬇️

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[nalimilan]

Thanks!