Revert "Add workaround for asset precompilation failures"

This workaroud breaks precompilation of assets in nested folders.

This reverts commit c0d75333c1985af942789cac23ebf089b6b65b1e.
Configure whitelisting of assets to be precompiled

Replace default Rails configuration Proc with our own that checks
against a whitelist of extensions.

Poorly-built Bower components often include a number of files not
needed at runtime. This solution has the added benefit of keeping
non-essential files  (.gzip .map .json .md .php .sh .txt) from being
served to users.

This is an alternative solution to the one found in commit c0d7533
See issue: https://github.com/sstephenson/sprockets/issues/347

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Fix issue causing all assets to be precompiled

Matchers specified in config.assets.precompile (also referred to as
'filters' in the Sprockets internals) are called on each asset in the
assets load path and should return a boolean expression.
The String used in ThemeFinder is truthy. Because matchers are
[OR'd][1], this was causing all assets to precompiled.

 * Use a lambda rather than Proc to ensure arity is checked.

[1]:https://github.com/sstephenson/sprockets/blob/2.x/lib/sprockets/base.rb#L421

Signed-off-by: Alex Coles <alex@alexbcoles.com>