fix: bashism

[e-kwsm]

The shebang of bin/installer.sh is #!/bin/sh but it is not POSIX-compatible:

• read -p: https://github.com/Shougo/dein.vim/blob/71550179444d3cbde5ac12f68b66e4b0ceb01a3e/bin/installer.sh#L144, https://github.com/Shougo/dein.vim/blob/71550179444d3cbde5ac12f68b66e4b0ceb01a3e/bin/installer.sh#L164
• &>: https://github.com/Shougo/dein.vim/blob/71550179444d3cbde5ac12f68b66e4b0ceb01a3e/bin/installer.sh#L194, https://github.com/Shougo/dein.vim/blob/71550179444d3cbde5ac12f68b66e4b0ceb01a3e/bin/installer.sh#L200

The simplest solution, I believe, is to use bash.

[samyilin]

No need to revert to Bash. We don't really need ➤ as a prompt, no need to revert to Bash because of this. &> can be replaced with >>/dev/null 2>&1, longer but is POSIX compliant, far as I know (Busybox ash does not complain, shellcheck didn't complain either). See Bash Pitfalls

[Shougo]

485 is better and it is merged.