v0.21.6

DataFrames v0.21.6

Diff since v0.21.5

Merged pull requests:

• make colnames type stable in broadcasting (#2331) (@bkamins)
• Don't specialize groupreduce! on result array element type (#2335) (@quinnj)
• Add check if input is a table before trying alternative constructors (#2341) (@quinnj)
• Update outdated documentation (#2343) (@bkamins)
• Change Travis from Julia 1.4 to 1.5 (#2345) (@bkamins)
• Update dataframerow.jl (#2353) (@tbeason)