Accelerate `rlx_prv_discover` provider

Introduce option `enable_shallow_app_discovery` which is disabled by
default.

When enabled searching for `*.app` files is done by using
`filelib:wildcard/1` and doesn't recursively traverse all directories
under `libs_dir` root dirs.

Instead its assumed that each directory on `libs_dir` list contains a
flat list of application dirs or is an app itself.
Updated erlydtl dep to a tagged version, since lastest version breaks API, thereby breaking dialyzer tests
Remove destination start_clean.sh before copying, since with R16 it is 0444, which makes the write fail. Fixes #120
Merge pull request #121 from jwilberding/next

Fixes #120
Reverting erlydtl to 0.8.0 after discussion in https://github.com/erlydtl/erlydtl/issues/126
Merge pull request #123 from jwilberding/next

Reverting erlydtl to 0.8.0 after discussion in https://github.com/erlydt...
fix double include_erts call with vm.args and sys.config config vars
Update erlydtl to v0.9.0
 Add R16B03, R16B03-1 and 17.0-rc1 to .travis.yml
Make relx be compatible with Erlang 17.0-rc1
Merge pull request #125 from kaos/erlydtl

Update erlydtl to v0.9.0
Merge pull request #126 from kiela/master

Make relx be compatible with Erlang 17.0-rc1
Set auto_escape to false for erlydtl compilation
Merge pull request #132 from puzza007/erlydtl-auto-escape-false

Set auto_escape to false for erlydtl compilation
by default do not do app and release discovery under output dir
Added a command line override for the sys_config setting.
update travis build command
unset REBAR target when building on travis; ensure latest rebar
setting REBAR to the rebar in the current directory works
Merge pull request #134 from nuex/travis_rebar

Ensure latest rebar is fetched when building on Travis-CI