Web UI not reachable after starting CLI

**Describe the bug**
After starting the binary, the web UI isn't reachable through the browser.
There are no errors evidently related to an UI issue.

**To Reproduce**
```
wget https://gdbgui.com/downloads/linux/gdbgui_0.13.1.1
chmod 755 gdbgui_0.13.1.1
./gdbgui_0.13.1.1
failed to initialize socketio app with async mode "gevent". Continuing with async mode "threading".
Opening gdbgui with default browser at http://127.0.0.1:5000
exit gdbgui by pressing CTRL+C
```


**Please complete the following information:**
* OS: Ubuntu 18.04
* gdbgui version (`gdbgui -v`): 0.13.1.1
* gdb version (`gdb -v`): 8.1.0.20180409-git
* browser [e.g. chrome, safari]: Firefox and Chrome
* python packages (`pip freeze`):

```
~$ pip3 freeze
apturl==0.5.2
asn1crypto==0.24.0
avocado-framework==66.0
avocado-framework-plugin-runner-remote==66.0
backcall==0.1.0
bcrypt==3.1.4
beautifulsoup4==4.6.3
Brlapi==0.6.6
bs4==0.0.1
certifi==2018.10.15
cffi==1.11.5
chardet==3.0.4
Click==7.0
command-not-found==0.3
cryptography==2.4.2
cupshelpers==1.0
decorator==4.3.0
defer==1.0.6
distro-info==0.18
docopt==0.6.2
epdb==0.15.1
Fabric3==1.14.post1
fancycompleter==0.8
flake8==3.5.0
Flask==1.0.2
greenlet==0.4.15
grip==4.5.2
httplib2==0.9.2
idna==2.7
ipaddress==1.0.22
ipython==7.1.1
ipython-genutils==0.2.0
itsdangerous==1.1.0
jedi==0.13.1
Jinja2==2.10
keyring==10.6.0
keyrings.alt==3.0
language-selector==0.1
launchpadlib==1.10.6
lazr.restfulclient==0.13.5
lazr.uri==1.0.3
louis==3.5.0
macaroonbakery==1.1.3
Mako==1.0.7
Markdown==3.0.1
MarkupSafe==1.1.0
mccabe==0.6.1
meld==3.18.0
msgpack==0.6.0
neovim==0.2.6
neovim-remote==2.1.2
oauth==1.0.1
olefile==0.45.1
paramiko==2.4.2
parso==0.3.1
path-and-address==2.0.1
pbr==5.1.1
pdbpp==0.9.3
pexpect==4.6.0
pickleshare==0.7.5
Pillow==5.1.0
prompt-toolkit==2.0.7
protobuf==3.0.0
psutil==5.4.8
ptyprocess==0.6.0
pyasn1==0.4.4
pycairo==1.16.2
pycodestyle==2.3.1
pycparser==2.19
pycrypto==2.6.1
pycups==1.9.73
pyflakes==1.6.0
Pygments==2.3.0
pygobject==3.26.1
pymacaroons==0.13.0
PyNaCl==1.3.0
pynvim==0.3.1
pyRFC3339==1.0
pyserial==3.4
python-apt==1.6.3
python-debian==0.1.32
pytz==2018.3
pyxdg==0.25
PyYAML==3.12
reportlab==3.4.0
requests==2.20.1
requests-unixsocket==0.1.5
SecretStorage==2.3.1
simplejson==3.13.2
six==1.12.0
stevedore==1.30.0
system-service==0.3
systemd-python==234
traitlets==4.3.2
ubuntu-drivers-common==0.0.0
ufw==0.35
unattended-upgrades==0.1
urllib3==1.24.1
usb-creator==0.3.3
virtualenv==15.1.0
wadllib==1.3.2
wcwidth==0.1.7
Werkzeug==0.14.1
wmctrl==0.3
xkit==0.0.0
zope.interface==4.3.2
```