Fix infinite scrolling issues.

[anlaital]

There were a couple of issues when using scrollToPage on a pager that had isInfiniteScrollEnabled set to true.

• .next used the wrong direction when animating from the last page to the first.
• .previous did not work at all when trying to scroll from the first page to the last.

[codecov]

Codecov Report

Merging #44 into master will decrease coverage by 0.57%.
The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   79.88%   79.31%   -0.58%     
==========================================
  Files          14       14              
  Lines         890      899       +9     
==========================================
+ Hits          711      713       +2     
- Misses        179      186       +7

Impacted Files	Coverage Δ
Sources/Pageboy/PageboyViewController.swift	74.45% <18.18%> (-2.69%)	⬇️

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[msaps]

Thanks! 😄 I'll get this in the next release 👍