Removed the default error-chain features
Removed the default error-chain features

On `x86_64-unknown-linux-musl` it looks like travis can't compile
the `backtrace-sys` crate because the `./configure` step fails.

The error message `./configure` gives is:

configure: error: in `/home/travis/build/azerupi/mdBook/target/x86_64-unknown-linux-musl/debug/build/backtrace-sys-204dc57c91e9a514/out':
configure: error: C compiler cannot create executables

azerupi/mdBook,azerupi/mdBook,azerupi/mdBook,azerupi/mdBook