Bump version to 0.55.0

[imaNNeo]

• FEATURE (by @emelinepal): Add tooltipBorder property in [LineTouchTooltipData], [BarTouchTooltipData], [ScatterTouchTooltipData], Border for Tooltip. #692.
• BUGFIX (by @imaNNeoFighT): Fix tooltip issue on negative bar charts, Stacked bar chart with both positive and negative values doesn't trigger tooltip when touching negative values #978.
• IMPROVEMENT (by @imaNNeoFighT): Use Container to draw axis-based charts border.
• FEATURE (by @FlorianArnould) Add the ability to select the RadarChart shape (circle or polygon), 如何实现网状雷达图?RadarChart (spider web chart) #1047.
• BUGFIX (by @imaNNeoFighT): Fix LineChart titles problem with single FlSpot, Throws exception if number of points on the x-axis = 1 #1053.
• FEATURE (by @FlorianArnould) Add the ability to rotate the RadarChar titles, Radar Chart: No rotated titles #883.
• BREAKING (by @FlorianArnould) RadarChartData.getTitle have a new parameter angle and now returns a RadarChartTitle instead of a simple string. (Read our Migration Guide to learn more about it)

[codecov]

Codecov Report

Merging #1067 (2837403) into master (4f7f98c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1067   +/-   ##
=======================================
  Coverage   86.32%   86.32%           
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  Files          45       45           
  Lines        2808     2808           
=======================================
  Hits         2424     2424           
  Misses        384      384           

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