Let's RxSwift 6.0

[tokijh]

• Update RxSwift to 6.0.0
• Bump deployment target to iOS 9 because RxSwift supports iOS9 and above only.
• Use Swift 5.1
• Use Xcode 12.2 on Travis

[codecov[bot]]

Codecov Report

Merging #17 (20719ff) into master (c3a3bb3) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #17   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines           78        85    +7     
=========================================
+ Hits            78        85    +7     

Impacted Files	Coverage Δ
Sources/RxExpect/RxExpect.swift	100.00% <100.00%> (ø)
Sources/RxExpect/Assertion.swift	100.00% <0.00%> (ø)

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