Import Plots in Pendulum environment

[raghav9-97]

Pendulum.jl uses Plots in line 83 without importing it which throws out an error on plotting annotations := [(0, -0.2, Plots.text("a: $(env.a)", :top))]

The error it throws out: ERROR: UndefVarError: Plots not defined

[iblislin]

Ref: #37