Export R_PROFILE during the configure step, rather than export

[jimhester]

export is run after configure, and R_PROFILE needs to be set for
setup_bioc.

It looks to me like the export stage is always run after configure, which is why the repos were not being set

travis-build/lib/travis/build/stages.rb

Lines 15 to 19 in 4f580b2

	Stage.new(:builtin, :configure, :always),
	Stage.new(:builtin, :prepare, :always),
	Stage.new(:builtin, :disable_sudo, :always),
	Stage.new(:builtin, :checkout, :always),
	Stage.new(:builtin, :export, :always),

An example error this should fix is https://travis-ci.org/ropenscilabs/available/jobs/452418452#L913-L916

cc @LiNk-NY

[LiNk-NY]

Thanks @BanzaiMan ! 🎉