MAINTENANCE : Updgraded supported Java version to 1.8

[carlwilson]

• bumped Java source and target to 1.8 in pom.xml; and
• .travis.yml : removed OpenJDK 7 & added Oracle JDK 9.

Closes #369

[codecov]

Codecov Report

Merging #342 into integration will decrease coverage by <.01%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##             integration     #342      +/-   ##
=================================================
- Coverage          44.49%   44.49%   -0.01%     
+ Complexity          3419     3418       -1     
=================================================
  Files                368      368              
  Lines              30330    30323       -7     
  Branches            5995     5988       -7     
=================================================
- Hits               13495    13491       -4     
+ Misses             14426    14423       -3     
  Partials            2409     2409

Impacted Files	Coverage Δ	Complexity Δ
...va/edu/harvard/hul/ois/jhove/module/XmlModule.java	37.07% <0%> (-0.14%)	18% <0%> (ø)
...main/java/edu/harvard/hul/ois/jhove/JhoveBase.java	40.88% <0%> (-0.13%)	41% <0%> (-1%)
...edu/harvard/hul/ois/jhove/module/pdf/AProfile.java	34.01% <0%> (-0.13%)	42% <0%> (ø)
...va/edu/harvard/hul/ois/jhove/module/PdfModule.java	70.94% <0%> (-0.02%)	292% <0%> (ø)
...u/harvard/hul/ois/jhove/module/wave/LinkChunk.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...edu/harvard/hul/ois/jhove/module/tiff/TiffIFD.java	37.73% <0%> (+0.02%)	189% <0%> (ø)	⬇️
...va/edu/harvard/hul/ois/jhove/module/GifModule.java	51.75% <0%> (+0.12%)	28% <0%> (ø)	⬇️

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