적분 $\displaystyle \int _ {- \frac{1} {2}} ^ {\frac{\sqrt {3} - 1} {2}} \frac{1} {x ^ {2} + x + 1} d x$ 의 값은?

• ①$\displaystyle \frac{\sqrt {3} \pi} {2}$
• ②$\displaystyle \frac{\sqrt {3} \pi} {3}$
• ③$\displaystyle \frac{\sqrt {3} \pi} {4}$
• ④$\displaystyle \frac{\sqrt {3} \pi} {5}$
• ⑤$\displaystyle \frac{\sqrt {3} \pi} {6}$