nvm: nvm use fails when NVM_DIR ends with a slash

nvm --version: v0.33.8
$SHELL: /bin/bash
$HOME: /home/bamboo
$NVM_DIR: '$HOME/.nvm/'
$PREFIX: ''
$NPM_CONFIG_PREFIX: ''
$NVM_NODEJS_ORG_MIRROR: 'https://nodejs.org/dist'
$NVM_IOJS_ORG_MIRROR: 'https://iojs.org/dist'
shell version: 'GNU bash, version 4.3.48(1)-release (x86_64-pc-linux-gnu)'
uname -a: 'Linux 4.4.0-109-generic #132-Ubuntu SMP Tue Jan 9 19:52:39 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux'
OS version: Ubuntu 16.04.3 LTS
curl: /usr/bin/curl, curl 7.47.0 (x86_64-pc-linux-gnu) libcurl/7.47.0 GnuTLS/3.4.10 zlib/1.2.8 libidn/1.32 librtmp/2.3
wget: /usr/bin/wget, GNU Wget 1.17.1 built on linux-gnu.
git: /usr/bin/git, git version 2.7.4
grep: /bin/grep, grep (GNU grep) 2.25
awk: /usr/bin/awk, GNU Awk 4.1.3, API: 1.1 (GNU MPFR 3.1.4, GNU MP 6.1.0)
sed: /bin/sed, sed (GNU sed) 4.2.2
cut: /usr/bin/cut, cut (GNU coreutils) 8.25
basename: /usr/bin/basename, basename (GNU coreutils) 8.25
rm: /bin/rm, rm (GNU coreutils) 8.25
mkdir: /bin/mkdir, mkdir (GNU coreutils) 8.25
xargs: /usr/bin/xargs, xargs (GNU findutils) 4.7.0-git
nvm current: v6.9.1
which node: $NVM_DIR/versions/node/v6.9.1/bin/node
which iojs:
which npm: $NVM_DIR/versions/node/v6.9.1/bin/npm
npm config get prefix: $NVM_DIR/versions/node/v6.9.1
npm root -g: $NVM_DIR/versions/node/v6.9.1/lib/node_modules