update to Android Studio 3.1.1

[codecov-io]

Codecov Report

Merging #77 into master will decrease coverage by 0.02%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #77      +/-   ##
============================================
- Coverage     69.32%   69.29%   -0.03%     
  Complexity     1190     1190              
============================================
  Files           107      107              
  Lines          4948     4954       +6     
  Branches        501      501              
============================================
+ Hits           3430     3433       +3     
- Misses         1193     1196       +3     
  Partials        325      325

Impacted Files	Coverage Δ	Complexity Δ
...dplot-core/src/main/java/com/androidplot/Plot.java	52.44% <0%> (-0.31%)	64% <0%> (ø)
...rc/main/java/com/androidplot/xy/XYGraphWidget.java	67.28% <0%> (-0.16%)	78% <0%> (ø)
.../main/java/com/androidplot/ui/FixedTableModel.java	93.75% <0%> (+0.13%)	6% <0%> (ø)	:arrow_down:
...c/main/java/com/androidplot/xy/SimpleXYSeries.java	78.03% <0%> (+0.33%)	32% <0%> (ø)	:arrow_down:

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[halfhp]

Thx for the contribution!