Change Main Makefile Targets

Based on feedback from @saivann, I changed the main Makefile targets to:

    ## `make` (no arguments): just build
    default: build

    ## `make test`: don't build, but do run all tests
    test: pre-build-tests post-build-tests

    ## `make valid`: build and run fast tests
    valid: pre-build-tests-fast build post-build-tests-fast

    ## `make all`: build and run all tests
    all: pre-build-tests build post-build-tests

No other unrelated changes in this commit, but a merge with @jm3's Ruby
bundler branch is planned, at which time the default JEKYLL_COMMAND will
change to use bundler exec.