housekeeping: Upgraded Xamarin.Forms to v4.2

[RLittlesII]

What kind of change does this PR introduce?

Upgrades Xamarin.Forms to v4.2

What is the current behavior?

Xamarin.Forms references to v4.1

What is the new behavior?

Xamarin.Forms references to v4.2

What might this PR break?

ReactiveUI.XamForms
ReactiveUI.Events.XamForms

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2225 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2225   +/-   ##
=======================================
  Coverage   55.86%   55.86%           
=======================================
  Files         114      114           
  Lines        4495     4495           
  Branches      688      688           
=======================================
  Hits         2511     2511           
  Misses       1803     1803           
  Partials      181      181

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fb1aea4...4ac0894. Read the comment docs.