Run Travis analysis/licences scripts under bash (#4665)

/bin/sh, for example, doesn't support pushd/popd.

travis/analyze.sh

+#!/bin/bash
 echo "Analyzing dart:ui library..."
 RESULTS=`dartanalyzer                                                          \
   --options flutter/analysis_options.yaml                                      \

travis/licenses.sh

+#!/bin/bash
 set -e
 shopt -s nullglob